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| title | date | lastmod | toc | comments | authors | categories | tags | ||||||
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| How to create (or remove) a swapfile on Linux | 2023-04-04T17:26:44-05:00 | 2023-04-04T17:26:44-05:00 | true | true |
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This is a step-by-step tutorial on how to create or remove a swapfile on a Linux machine, and whether you should create swap space in the first place.
Also included is the most correct fstab entry: most articles get this "wrong"
(but it doesn't actually matter).
Should you use swap?
A common misconception is that adding swap to your system can reduce performance. However, even if you aren't running out of RAM, it can still be beneficial to add swap.
See these points according to an article by Hayden James:
Even if there is still available RAM, the Linux Kernel will move memory pages that are hardly ever used into swap space.
It’s better to swap out memory pages that have been inactive for a while, keeping often-used data in cache, and this should happen when the server is most idle, which is the aim of the Kernel.
Avoid setting your swap space too large if it will result in prolonging performance issues, outages, or your response time (without proper monitoring/alerts).
Should you use a swap file or partition?
Swap partitions should be preferred because swapfiles tend to be slower and slightly more complex to set up, especially if hibernating. A swapfile might be preferred due to its flexibility (ease to resize), but if you use LVM then you can easily resize the swap partition anyways.
Tutorial
{{< include path="include/console.md" markdown=true >}}
Step 1: Create the file
The first step is to allocate the file.
# dd if=/dev/zero of=/swapfile bs=1M count=[size in MiB] status=progress
Replace [size in MiB] with the size of your swapfile in Mebibytes (MiB). You
can use this online converter to
convert from Gibibytes (GiB, often confused with Gigabytes, but that's a
misconception for another time) to MiB, which you can put in the command.
Or you can look at this table for common sizes:
{{< table >}}
| GiB | MiB |
|---|---|
| 1 | count=1024 |
| 2 | count=2048 |
| 3 | count=3072 |
| 4 | count=4096 |
| 8 | count=8192 |
| 16 | count=16384 |
| {{< /table >}} |
If you aren't sure how big your swapfile should be, take a look at Table 15.1 on this Red Hat documentation page. Remember that if your first swapfile isn't large enough, you can create another one.
On my computer I have 32 GiB of RAM and a 16 GiB swap partition, rather than swapfile, because I can flexibly expand the swap partition using LVM.
{{< aside example >}}
To create a swapfile 4 GiB in size, you would run:
# dd if=/dev/zero of=/swapfile bs=1M count=4096 status=progress
4096+0 records in
4096+0 records out
4294967296 bytes (4.3 GB, 4.0 GiB) copied, 0.960183 s, 4.5 GB/s
{{< /aside >}}
Step 2: Change swapfile permissions
The swapfile should only be readable by the system (root user). Run this
command to change it:
# chmod 600 /swapfile
Step 3: Format the swapfile
Use the mkswap command to format the file to be used as swap (basically just
add a header to identify it):
# mkswap /swapfile
Setting up swapspace version 1, size = 4 GiB (4294963200 bytes)
no label, UUID=a0b87eca-b951-4344-be2d-020d77cdef48
Step 4: Create an entry in /etc/fstab
An entry needs to be added to /etc/fstab for the swapfile to be enabled during
bootup. Open /etc/fstab in a text editor, and add this line to the end (add
spaces to line up with other entries if you want, whitespace is ignored):
/swapfile none swap sw 0 0
{{< aside note >}}
Above, I have filled the "options" field with sw. Some guides suggest using
either sw or defaults. Neither of those options is valid for swapon and
they are both ignored. However, the field does need to be filled out with
something, so feel free to put something funny (do let me know in the comments
if this somehow breaks something though).
If you're curious, this is my fstab entry (UUID is because I use a swap
partition, rather than swapfile):
# /dev/mapper/bobavg0-swap
UUID=4f7c3ae8-839b-4474-b8a5-96bd78db06f8 none swap bobaswap 0 0
{{< /aside >}}
Step 5: Enable the swapfile
Adding the fstab entry won't enable the swapfile until a reboot. To enable it
now, use the swapon command.
# swapon /swapfile
Finally: check the swap status
Use swapon and free to verify that your new swapfile has been added:
$ swapon --show
NAME TYPE SIZE USED PRIO
/swapfile file 4G 0B -2
$ free -h
total used free shared buff/cache available
Mem: 31Gi 3.6Gi 20Gi 233Mi 6.9Gi 26Gi
Swap: 4.0Gi 0B 4.0Gi
{{< aside note >}}
If you get an error saying that the swapon command was not found, try running
it as root (using sudo). On Debian-based distributions the swapon command
is not available to regular users.
$ swapon --show
bash: swapon: command not found
# swapon --show
NAME TYPE SIZE USED PRIO
/swapfile file 4G 0B -2
{{< /aside >}}
Removing a swapfile
{{< aside warning >}}
Be careful that the swapfile isn't being highly used because once you run
swapoff, it will dump the entire contents back into RAM and may cause you to
run out of memory.
{{< /aside >}}
Step 1: Disable/unload the swapfile
$ swapon --show
NAME TYPE SIZE USED PRIO
/swapfile file 4G 0B -2
$ # the path to the swapfile I want to remove is `/swapfile`
# swapoff /swapfile
Step 2: Remove the entry from fstab
Open /etc/fstab in a text editor and find the line matching the swapfile you
want to remove, and delete it. For example:
/swapfile none swap sw 0 0
Step 3: Delete the actual file
# rm /swapfile
References
Use dd instead of fallocate
See "Files with holes" under the man page.
The options field in fstab
On Linux, sw doesn't mean anything for swapon, so it gets ignored. See: